The present invention relates to a method of firing in a boiler, in which method loosely stacked fuel is introduced through a charging opening into the furnace of the boiler on a first support, and jets of ignition air entraining hot flue gas from the furnace are directed at the surface of the loosely stacked fuel on the support so that the surface layer of the loosely stacked fuel is ignited and the fuel is partially gasified, whereupon the fuel is passed on to a grate located at a lower level on which the final combustion of the fuel takes place. The present invention also relates to a boiler for using the method.
In prior-art boilers, the area of the furnace at the charging opening is formed so as to be relatively closed, being defined upwards by a slightly inclined portion of the wall above the charging opening with inserted ignition air nozzles and downwards by a fixed bottom inclined at an angle of less than 5xc2x0 to 20xc2x0. This prior-art structure is designed to allow control of the supply of oxygen and thus the possible release of energy for expulsion of gases from the loosely stacked fuel on the fixed bottom. It has proved, however, that it is difficult to obtain a stable rate of combustion because the combustion can vary strongly at relatively small variations of the air permeability and humidity content of the loosely stacked fuel and the rate at which the fuel is charged into the furnace.
It is a further disadvantage of the prior-art boiler with a fixed bottom that deposits of melted or sintered ash particles can accumulate partly on the boiler wall in the area around the ignition air nozzles, partly on the first support. To ensure the free passage of the fuel it is necessary to remove such deposits at regular intervals, which may cause undesirable stop-pages. Prior art also suffers from the defect that the combustion develops a not inconsiderable amount of nitrogen oxide (NO), which is emitted to the surroundings and pollutes the environment via the chimney of the boiler plant.
U.S. patent publication Ser. No. 4,213,405 describes a boiler for combustion of solid fuel, such as wood waste, in which the furnace of the boiler has a first support in the form of an inclined grate the main purpose of which is to dry the fuel, the flue gases from the combustion zones above a second grate located at a lower level being conducted to and passing up through a top zone of the first grate.
The object of the present invention is to provide a method which reduces or eliminates the above problems. This is obtained in accordance with the invention by a method of the type mentioned in the introduction and characterized in that the air from the ignition air jets are permitted, together with the entrained flue gas, to pass down through the loosely stacked fuel and the first support and then to flow off to the furnace, and that the flue gas entrained by the ignition air jets is drawn substantially from a section of the furnace through which flows a mixture of combustion products from the first support and the grate.
By means of the method, the air from the ignition air nozzles together with the entrained flue gas passes through the fuel to the first support, in the following called the first grate, and then t through the grate to the lower side thereof. The result of this is that the gases developed by ignition of the surface layer of the fuel and the gasification of the underlying part of the fuel are passed on down through the fuel to the lower side of the first grate. The combustion gases flow from here out into the furnace and thus pass above the combusting fuel located on the second, lower grate, whereby the combustion gases from the two grates are mixed while being mixed with primary and secondary air supplied to the furnace in a conventional manner. Part of this mixture of flue gases is then drawn in by the ignition air jets from the furnace towards the surface of the fuel on the first support.
Surprisingly, this method has turned out to provide extremely stable combustion, and the rate of combustion can easily be controlled regardless of any variations of the air permeability or humidity content of the loosely stacked fuel or its feeding rate.
It has furthermore proved that the method according to the invention results in substantially smaller amounts of melted or sintered ash particles, and a not inconsiderable reduction of the amount of nitrogen oxide produced by the combustion is seen.
Finally, it can be mentioned that no backfire of the combustion to the charging channel occurs in the boiler according to the invention, as is the case in the prior-art boiler.
It is assumed that these good results are due, mainly, to the fact that the mixture of air and flue gas that hits the surface of the loosely stacked fuel together with the gasification products from the loosely stacked fuel is passed down below the first grate. This ensures that hot flue gas from the furnace can flow into the area around the ignition air nozzles, that a large proportion of this flue gas is recirculated through the loosely stacked fuel, and that the gases around the surfaces of the first grate have a relatively homogeneous and low temperature. This must be seen in relation to the prior-art boiler, in which the ignition air jets are unable to penetrate the fuel bed as the bottom under the fuel bed is not open to penetration. This means that the products developed by the gasification, which are relatively cold compared with the flue gas in the middle of the furnace, escape from the upper surface of the loosely stacked fuel substantially to the area around the ignition air nozzles and thus limit the flow of hot flue gas to this area. This fact is further supported in the prior-art boiler by the wall above the charging opening being formed with a near-horizontal inclination, thus preventing the flow of flue gas from the middle of the furnace.
The prior-art boiler with a fixed bottom has no well-defined flow pattern at the surface of the fixed bottom. This allows the hot gas flow from the ignition air nozzles to follow randomly occurring holes in the fuel bed and to hit the fixed bottom, whereby partially melted particles of ash suspended in the hot flue gas can be deposited on the bottom. This is avoided to a substantial extent in the method according to the invention, partly because such flows seek towards the openings in the underlying first grate and thus do not deposit particles on the grate surface, partly because the remaining part of the grate surface supporting the overlying fuel is protected by the fuel to a higher degree.
The prior-art boiler with a fixed bottom has a large excess of combustible gases and thus a deficit of oxygen in the area around the ignition air nozzles. Consequently, the amount of air and thus of oxygen supplied to the area around the ignition air nozzles has a crucial influence on the rate at which the gasification products are expelled from the fuel on the fixed bottom, as the supply of air to the area around the ignition air nozzles will cause a strong temperature increase in the area under these conditions. As a substantial part of the gasification of the fuel on the fixed bottom typically occurs at temperatures below 500xc2x0 C., whereas oxidation of these gasification products requires temperatures above 800-1,000xc2x0 C., a careful balancing of the relationship between the amount of air supplied to the flue gas at the area around the ignition air nozzles and the degree of heat transfer between flue gas and fuel on the fixed bottom is required. In case of excessive heat transfer, the temperature of the flue gas at the ignition air nozzles will drop, whereby the reaction between combustible components of the flue gas and oxygen supplied by the air will cease. In case of an insufficient heat transfer the temperature in the flue gas entrained by the ignition air jets will rise to a level where ash particles suspended in the flue gas will melt and where ash formed in the surface of the fuel bed will melt and thus further inhibit the heat transfer between flue gas and fuel. To control these conditions, the ignition air nozzles in the prior-art boiler are arranged in a relatively closed area of the furnace.
The method according to the invention avoids these problems because flue gas in which most of the combustible products are oxidized is drawn for the ignition air nozzles. As a substantial part of the chemically bound energy has already been transformed into heat in the flue gas before the latter is mixed with the ignition air, the temperature will thus always be sufficient to ensure continued oxidation in the flue gas during the mixing itself. The reduced amount of combustible gases in the flue gas around the ignition air nozzles also means that there is more oxygen present in the mixture of ignition air and flue gas so that the amount of air supplied with and around the ignition air jets does not have the same decisive influence on the latters"" temperature as in the prior art described. This makes it possible for the amount of ignition air supplied in the ignition air jets to be varied considerable more freely, exclusively for the purpose of obtaining the desired rate of combustion on the first grate.
A further effect of the excess of oxygen mentioned above in and around the ignition air jets in combination with the flow through the fuel bed according to the invention is that the nitrogen compounds contained in the flue gas at the ignition air nozzles and deriving from the fuel will be oxidized in the ignition air jets and be able to react with non-oxidized nitrogen compounds in the gasification products with formation of free nitrogen, which is not to any substantial degree oxidized into nitrogen oxide in the furnace. This results in a substantial reduction of the total production of nitrogen oxide at the combustion. It is assumed that the formation of free nitrogen is promoted by the increased presence of free radicals, as will be described below.
To efficiently avoid the melting of ash particles suspended in the flue gas and their fastening on the walls of the boiler around the ignition air nozzles it is necessary to restrict the temperature in this area to 900-1,100xc2x0 C. However, the method according to the invention permits a temperature in the ignition air jets themselves which is higher than the above level because according to the method, the ignition air jets pass through the first support contrary to the ignition air, which is xe2x80x9cthrown backxe2x80x9d in the prior-art boiler and thus hits the area around the ignition air nozzles.
In the method according to the invention, the ignition air and entrained flue gas will be cooled at the passage of the fuel bed as a consequence of the mixing with gasification products.
The above higher temperature permitted in the ignition air jets according to the invention contributes to efficient decomposition of the tar substances in the gasification products. It is assumed that the decomposition of tar substances is further promoted if CO and H2 are combusted immediately before the flue gases entrained by the ignition air jets meet the fuel bed, as the following rapid chain reactions will occur:
H2+OHxe2x86x92H2O+H
CO+OHxe2x86x92CO2+H
O2+Hxe2x86x92O+OH
O+H2Oxe2x86x922OH
resulting in the following net reactions:
H2+O2xe2x86x922OH
CO+O2+H2Oxe2x86x92CO2+2OH
The free OH radicals so formed can react with the tar substances and contribute to their decomposition. Alternatively, they will disintegrate through, for example, the following reactions:
CO+OHxe2x86x92CO2+H
OH+Hxe2x86x92H2O
resulting net in the following reaction:
CO+2OHxe2x86x92CO2+H2O.
It must then be assumed that the decomposition of tar substances is promoted if the flue gas drawn in towards the ignition air nozzles from the furnace contains considerable amounts of CO and H2. Under the normal conditions in a furnace where the flue gas may be considerably inhomogeneous, this will be the fact if the flue gas, before it is entrained by the ignition air, is supplied with 60-110 per cent, preferably 70-100 per cent, especially 80-90 per cent, of the amount of air stoichiometrically required for combustion of the fuel. These mixing ratios are correspondingly assumed to be nearly optimum to obtain the reduction of the formation of nitrogen oxide described above.
In the boiler according to the invention, these mixing ratios can be maintained by tertiary air nozzles being placed in at least one of the boiler walls at a level with or above a narrowing of the flow cross-section of the furnace formed by the inclined wall above the charging opening. The tertiary air is necessary to ensure that the flue gas coming from the furnace below the narrowing is supplied with so much oxygen that the flue gas can be completely combusted before it is passed on to the convection part of the boiler. The location mentioned ensures that the tertiary air supplied is not mixed with the flue gas flowing in to the ignition air nozzles and thus changes the advantageous mixing ratios mentioned above.
The fact that the gasification products from the first grate are passed over and mixed with the combustion products from the second grate means partly that the gasification products cannot reach the ignition air jets in a pure form and thus, as a consequence of their content of tar substances, impede the oxidation of the jets, partly that the tar substances are further decomposed, as the combustion products from the second grate to a greater extent consist of CO, which decomposes the tar substances at co-combustion therewith, as already described.
In another embodiment of the method according to the invention, the combustion can be adjusted by increasing the ignition air amount when the oxygen content of the flue gas is high or is increasing and by reducing the ignition air amount when the oxygen content of the flue gas is low or is decreasing. This form of adjustment, known per se, has proved to become very stable when used in connection with the method according to the invention, as undesired influence from gasification products returning straight from the surface of the fuel bed to the ignition air jets has now been eliminated.
The present invention also relates to a boiler for using the method described above, the boiler comprising a furnace with a charging opening, a first support formed as a first grate with grate apertures, a grate located at a lower level, and ignition air nozzles located above the first support, which boiler is characterized in that said grate apertures constitute more than 20 percent, preferably more than 50 percent, especially more than 70 percent of the area of the first grate, that the ignition air nozzles are located in a wall of the boiler above the charging opening and are directed at the first grate, and that the wall above the charging opening is inclined upwards towards the middle of the furnace at an angle with horizontal of 20-70xc2x0, preferably 30-60xc2x0, especially 45xc2x0.
In the embodiment described, the design of the front wall inclined upwards ensures that the flue gas is drawn in from the particular section of the furnace located at the charging opening through which the desired mixture of flue gas from the two grates will flow.
Further embodiments are disclosed herein.